The present invention relates to the magnetic resonance arts. It finds particular application in medical diagnostic imaging and will be described with reference thereto. However, it will be appreciated that the invention will also find application in other types of imaging, spectroscopy, and the like.
A typical magnetic resonance (MR) imaging sequence includes an RF excitation pulse, e.g. a 90xc2x0 pulse, with a corresponding slice- or slab-selective magnetic gradient pulse, followed by a series of spatial encoding and readout magnetic field gradient pulses. In some sequences a second 180xc2x0 refocusing pulse is applied between the initial excitation pulse and the spatial encoding/readout region. The 180xc2x0 pulse effectively reverses the dephasing effect of small spatial variations in the MR frequency due to spatial variations in the applied magnetic field, and refocuses the magnetization to form a spin-echo. MR imaging is performed using various imaging modes which usually vary with respect to the method and timing of the spatial encoding and data readout sequences.
The choice of spatial encoding and data readout scheme has significant consequences on the imaging contrast, resolution, and scanning speed. Two imaging parameters are the time-to-echo, TE, and the repeat time between RF excitations, TR. Sampling the induced resonance nearer to the excitation emphasizes proton density weighting or T1 weighting in which the contrast strongly reflects the regrowth rate of the MZ component of the net magnetization. Sampling the magnetization later emphasizes T2 weighting in which the contrast strongly reflects the decay rate of the MXY, component of the net magnetization.
Proton density (xcfx81) weighting is obtained when the TE delay is short and the magnetic resonance has minimal time to decay, so that the density of resonant hydrogen protons is measured. T2* weighting is obtained using a longer TE delay so that the fastest (T2*) magnetic resonance decay is a factor. The T2* decay differs from T2 in that T2* includes inhomogeneous dephasing due to static magnetic field inhomogeneities. To measure the xe2x80x9cpurexe2x80x9d T2 corresponding to dephasing due to molecular interactions (excluding inhomogeneous dephasing), a 180xc2x0 RF refocusing pulse is applied to induce a spin-echo during the sampling interval. Other types of pre-pulses can also be applied to provide fat suppression, MTC, et cetera.
Prior spatial encoding and readout schemes have been configured to provide a variety of xcfx81, T2, or T2* weightings. The choice of spatial encoding scheme strongly affects the scan speed and resolution. A popular MR imaging mode is echo-planar imaging (EPI). In the EPI imaging mode, an oscillating read gradient generates a series of gradient echoes. Phase encoding pulses between echoes step the sampling through k-space in a back-and-forth rastering fashion. The speed of EPI is preferably sufficient that the k-space data for an entire planar (slice) image is obtained from a single RF magnetic resonance excitation, i.e. xe2x80x9csingle-shotxe2x80x9d EPI, or SS-EPI. The rapidly switched gradients along with a rastered readout timing sequence of SS-EPI produce complete slice scans in as little as a few hundred milliseconds or less. This speed makes SS-EPI an ideal method for clinical imaging when short scan times are important. Reduced scan times translate to reduced image blurring due to patient movements, respiration, cardiac action, and the like.
The EPI technique encompasses a number of variants, including several techniques collectively known to the art as partial parallel imaging (PPI). In the PPI techniques, a phased array receive coil simultaneously measures the MR response using a plurality of phased receive coils and combines the data from the array to acquire a plurality of k-space samples in parallel.
Enhancement in MR imaging can also be obtained through the use of multiple image techniques. In these methods, the spatial encoding scheme is designed so that multiple images, typically using more than one image contrast mode, are obtained from the echo train following a single RF excitation pulse. For example, a T2* weighted image and a T2 weighted image can be obtained.
Another type of MR imaging is contrast-enhanced imaging. In this type of MR imaging, a magnetic contrast agent, such as a gadolinium chelate, is administered to the patient, such as by a bolus injection. The magnetic contrast agent provides enhanced MR contrast versus intrinsic imaging. In some studies, the preferential concentrating of the contrast agent in particular organs or tissues is imaged. In vascular imaging, the distribution of an administered contrast agent is monitored over time to study the performance of major blood vessels. Similarly, the perfusion of the contrast agent through tissues or organs enables study of the capillary performance in the targeted areas.
In order to quantitatively analyze perfusion by contrast-enhanced MR imaging, it is useful to quantify the concentration of the contrast agent in the imaged area based upon the MR image. In the exemplary case, the gadolinium chelate strongly reduces the T2 weighted signal and T2* weighted signal. In a T2 weighted MR image, the areas of high gadolinium chelate concentration appear darker than the surrounding areas. In principle, therefore, the contrast agent concentration can be extracted from the percentage darkening or from similar quantitative image analysis. Unfortunately, competing effects, such as brightening due to T1 shortening, can counteract the T2 darkening effect of the gadolinium chelate and produce errors in the quantitative analysis.
The prior art also discloses taking a reference image prior to administration of the contrast agent. This approach has the disadvantage that the image of the contrast agent usually needs to be registered spatially with the reference image to correct for patient movement or other spatial shifting.
An effective method is needed for correcting these errors in quantitative contrast-enhanced perfusion imaging. Such correction would preferably utilize additional non-T2 weighted images to account for extraneous, non-T2 contrast mechanisms. However, the collection of these additional images is limited by the time constraints imposed by the dynamic perfusion process. The present invention contemplates a new imaging method which overcomes these limitations and others.
According to one aspect of the invention, a method of magnetic resonance imaging is disclosed. A magnetic resonance contrast agent is administered to a subject, which contrast agent alters T2 and T2* magnetic resonance characteristics. A magnetic resonance is excited in a region of interest of the subject which receives the contrast agent. A first echo planar image readout waveform is applied which generates first image data. After the first echo planar image readout waveform, a second echo planar image readout waveform is applied and a T2 or T2* weighted image data is generated. The image data is reconstructed to generate a proton density weighted or a T1 weighted image representation and a T2 or T2* weighted image representation. The T2 or T2* weighted image representation is corrected with the first image representation.
Preferably, the method includes applying an RF inversion pulse between the first and second echo planar image readout waveforms.
The method preferably includes applying a third echo planar image readout waveform and generating the other of T2 and T2* weighted image data. Optionally, an RF inversion pulse is applied between the second and third echo planar image readout waveforms, such that the second echo planar image readout waveform generates T2* weighted data and the third image readout waveform generates T2 weighted data. The T2 weighted data is preferably reconstructed into a T2 weighted image representation, and the T2 weighted image representation is preferably modified with the first image representation.
The method preferably includes reconstructing the T2 or T2* weighted image data and a portion of the first image data to generate the T2 or T2* weighted image representation, and reconstructing a portion of the T2 or T2* weighted image data and the first image data to generate the first image representation. Optionally, the portion of the T2 or T2* weighted data used in generating the first image and the portion of the first image data used in generating the T2 or T2* weighted image include interleaved data lines adjacent an edge of k-space. Optionally, additional data lines are generated by conjugate symmetry.
Preferably, the method includes repeating steps of the method a plurality of times to generate a series of first image representations and a series of T2 or T2* weighted image representations. These image series are preferably combined to generate a third series depicting a temporal evolution of the contrast agent in the region of interest.
Preferably, the method includes combining the first image representation and the T2 or T2* weighted image representation to generate a third image representation, and then repeating steps of the method a plurality of times to generate a series of third image representations depicting a temporal evolution of the contrast agent in the region of interest.
In the method, the contrast agent is preferably a gadolinium chelate.
At least one of the steps of generating the first image data and generating the second image data optionally advantageously includes generating image data using a partial parallel imaging technique.
According to another aspect of the invention, a method of contrast enhanced magnetic resonance imaging is disclosed. A subject is injected with a contrast agent, magnetic resonance is excited in a region of interest, the excited magnetic resonance is permitted to decay for a preselected duration to optimize one of T2 and T2* weighting, and after the preselected duration an echo planar image readout waveform is applied to generate T2 or T2* weighted data. The method further includes, during the preselected duration, applying another echo planar image readout waveform to generate T1 weighted data.
According to yet another aspect of the invention, a method is disclosed for imaging a patient using a magnetic resonance (MR) imaging apparatus. The MR apparatus includes a patient support means, a main magnet, a slice-select gradient pulse generator, a phase-encode gradient pulse generator, a read gradient pulse generator, a plurality of RF coils, an RF transmitter, and a receiver. The method includes the steps of: administering a contrast agent to the patient; exciting a magnetic resonance in the patient using the RF transmitter and at least one of the plurality of RF coils in conjunction with the slice-select gradient generator; encoding and reading the magnetic resonance using the phase encode and the read gradient generators in conjunction with at least one of the plurality of RF coils and the receiver, the encoding and reading implementing a first echo-planar image readout waveform; encoding and reading the magnetic resonance using the phase encode and the read gradient generators in conjunction with at least one of the plurality of RF coils and the receiver, the encoding and reading implementing a second echo-planar image readout waveform; and reconstructing the read, encoded, magnetic resonance into first and second image representations.
Preferably, the method further includes comparing the first image representation with the second image representation to obtain a third image representation thereby. Optionally, the method includes repeating the steps of exciting a magnetic resonance, encoding, reading, and reconstructing first and second images, and comparing the first images with the second images to obtain third images thereby. A temporal evolution of at least one of the first, second, and third images is determined.
In the step of reconstructing the second image, a portion of the phase and frequency encoded resonance from the first echo planar image readout waveform is preferably reconstructed into the second image.
In the method, the first echo planar image sequence phase encoding preferably includes phase encoding a first portion of the resonance such that a ky component single-steps in a first direction, and phase encoding a second portion of the resonance such that the ky component double-steps in the first direction. The second echo planar image readout waveform phase encoding preferably includes phase encoding a first portion of the resonance such that the ky component double-steps opposite to the first direction, and phase encoding a second portion of the resonance such that the ky component single-steps opposite to the first direction. The reconstructing step preferably includes reconstructing the first and second portions of the first echo planar sequence and the first portion of the second echo planar sequence into the first image representation, and reconstructing the second portion of the first echo planar sequence and the first and second portions of the second echo planar sequence into the second image representation.
According to still yet another aspect of the invention, a magnetic resonance imaging apparatus is disclosed. A main magnet generates a temporally constant magnetic field through an examination region. An RF system excites and manipulates magnetic resonance in the examination region and receives and demodulates magnetic resonance signals from the examination region into data lines. A sorter is provided for sorting the data lines between a first data memory and a second data memory. A gradient magnetic field system generates magnetic field gradients across the examination region to spatially encode the resonance signals. A sequence controller: (i) controls the RF system to induce resonance; (ii) controls the RF and gradient systems to implement a first echo planar readout waveform which generates T1 weighted data lines; (iii) controls the RF and gradient systems to implement a second echo planar readout waveform which generates one of T2 and T2* weighted data lines, and (iv) controls the sorter to sort the T1 and T2 or T2* weighted data lines between the first and second data memories. A reconstruction processor reconstructs data lines from the first data memory into a first image representation and data lines form the second data memory into a second image representation.
The magnetic resonance apparatus preferably further includes a means for injecting a contrast agent into a subject in the examination region, and an image processor for combining the first and second image representations into a contrast agent enhanced image representation. Optionally, the sequence controller controls the sorter to sort all of the T1 weighted data lines and a portion of the T2 or T2* weighted data lines into the first image memory, and all of the T2 or T2* weighted data lines and a portion of the T1 weighted data lines into the second image memory.
Preferably, the RF system further includes a phased array receive coil, and a partial parallel imaging (PPI) integrator which processes the readout of the phased array receive coil to generate data lines. The PPI integrator preferably processes the readout of the phased array receive coil using one of a simultaneous acquisition of spatial harmonics (SMASH) technique, a sensitivity encoding (SENSE) technique, and a parallel imaging with localized sensitivities (PILS) technique.
One advantage of the present invention is that it facilitates correction of extraneous MR effects.
Another advantage of the present invention is that it facilitates faster scan times.
Another advantage of the present invention is that it provides multiple images with complementary dynamic range.
Another advantage of the present invention is that it facilitates quantitative contrast-enhanced imaging.
Another advantage of the present invention is that it corrects for extraneous image contrast in perfusion imaging.
Yet another advantage of the present invention is that it separates out counteracting MR effects in contrast enhanced T2 weighted SS-EPI imaging.
Still yet another advantage of the present invention is that it provides additional MR data for a given imaging time that can be processed or combined to obtain improved and/or additional diagnostic information versus the prior art.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiment.